Atmospheric oxygen was considerably lower in Earth's geological past than previously assumed, scientists have found.
The findings also challenge theories that high levels of oxygen in the atmosphere may have caused dinosaurs to grow gigantic in size.
An international team of researchers led by Ralf Tappert, University of Innsbruck in Austria, reconstructed the composition of Earth's atmosphere of the last 220 million years by analysing modern and fossil plant resins.
The mineralogist and his colleagues from the University of Alberta in Canada and universities in the US and Spain produced a comprehensive study of the chemical composition of Earth's atmosphere since the Triassic period.
The research team analysed a total of 538 amber samples from well-known amber deposits worldwide, with the oldest samples being approximately 220 million years old and recovered from the Dolomites in Italy.
The team also compared fossil amber with modern resins to test the validity of the data.
The results of this comprehensive study suggest that atmospheric oxygen during most of the past 220 million years was considerably lower than today's 21 per cent.
"We suggest numbers between 10 and 15 per cent," said Tappert.
These oxygen concentrations are not only lower than today but also considerably lower than the majority of previous investigations propose for the same time period.
For the Cretaceous period (65 to 145 million years ago), for example, up to 30 per cent atmospheric oxygen has been suggested previously.
The researchers also related this low atmospheric oxygen to climatic developments in Earth's history.
"We found that particularly low oxygen levels coincided with intervals of elevated global temperatures and high carbon dioxide concentrations," explained Tappert.
Tappert suggests that oxygen may influence carbon dioxide levels and, under certain circumstances, might even accelerate the influx of carbon dioxide into the atmosphere.
According to the results of the study, oxygen may indirectly influence the climate. This in turn may also affect the evolution of life on Earth. A well-known example are dinosaurs: Many theories about animal gigantism offer high levels of atmospheric oxygen as an explanation.
Tappert now suggests to reconsider these theories: "We do not want to negate the influence of oxygen for the evolution of life in general with our study, but the gigantism of dinosaurs cannot be explained by those theories."
The study was published in the journal Geochimica et Cosmochimica Acta.